Electronic fuel control and safety shut-off system



June 23, 1959 B. MATTHEWS 2,891,729

ELECTRONIC FUEL CONTROL AND SAFETY SHUT-OFF'SYSTEM Filed 'Nov. 12, 1953 i 2 Sheets-Sheet 1 INVENTOR. R usse ll 1?. Mai'i'hewzs BY [AMI 01244::

Jun 23, 1959 'R. B. MATTHEWS 2,891,729

ELECTRONIC FUEL CONTROL AND SAFETY SHUT-OFF SYSTEM Filqd NOV. 12, 1953 2 Sheets-Sheet 2 INVENTOR.

, 2,391,729 Patented June 23, 1959 ELECTRONIC FUEL CUNTROL AND SAFETY SHUT-OF F SYSTEM Russell B. Matthews, Wanwatosa, Wis., assignor to Baso Inc, a corporation of Wisconsin Application November 12, 1953, Serial No. 391,504

4 Claims. (Cl. 236-9) This invention relates, in general, to burner control systems, and has particular relation to burner control systems of the type in which an electric circuit for an electroresponsive valve in the fuel supply pipe for a main burner is under the control of a thermoelectric generator subject to the heat of a pilot burner.

The present application is a continuation-impart of the copending application for patent of Russell B. Matthews, case 3, Serial No. 41,129, filed July 28, 1948 now Patent No. 2,682,016.

While the particular systems which I shall describe hereinafter in connection with the drawings are, in general, of the type in which there is under the control of the thermoelectric generator an electroresponsive valve in the fuel supply pipe for a main burner combined with a thermoelectric safety shut-off valve for the fuel supply for the pilot burner whereby 100 percent shut-01f may be secured, it is to be understood that the invention in its broader aspects is not limited to this particular type of system, but may be embodied in other systems as suitable and desired.

One of the main objects of the present invention is to provide an improved burner control system wherein means is provided for utilizing a thermoelectric generator subject to the heat of the pilot flame to control the flow of current through an electron tube, the output of which is used to control the circuit for the electroresponsive valve in the fuel supply pipe for the main burner.

Another and more specific object of the invention is to provide an improved burner control system wherein the flow of current through the electron tubethe output of which controls the circuit for the electroresponsive valve in the fuel supply pipe for the main burneris controlled (1) by passing current generated by the thermoelectric generator through a coil located on the outside of and in juxtaposition to the electron tube, or (2) by utilizing in a system of the class described a conventional vacuum tube containing a control grid and applying the voltage generated by the thermoelectric generator to the control grid.

Another object of the invention is to provide in a burner control system of the class described means which is operated by the output of the electron tube to open and hold open the electroresponsive valve in the fuel supply pipe for the main burner when the pilot burner is ignited, and in which extinguishment of the pilot flame cuts off the flow of current through the electron tube, thus deenergizing the electroresponsive valve which operates to shut off the supply of fuel to the main burner.

Another object of the invention is to provide in a burner control system of the class described means for controlling the electroresponsive valve Without mechanical contacts or the like.

Another object of the invention is to provide in a control system of the aforementioned character condition responsive means for controlling the flow of current through an electron tube and hence through the circuit controlled by the flow of current through said tube.

Still another object of the invention is to provide a control system of the character described wherein there is a modulating type control device controlled by the flow of current through the electron tube, said device having a flow-controlling member which is modulated between a first and a second position in accordance with the condition.

Further objects and advantages of the invention'will be apparent from the following detailed description, taken in connection with the accompanying drawings in which Figure l is a diagram showing schematically the parts and circuit connections of one form of system embodying the present invention;

Figure 2 is a perspective diagrammatic view of the magnetically controlled diode of the circuit shown in Figure 1 and the contactless thermostatic temperature controlling means associated therewith;'

Figure 3 is a diagram similar to Figure 1, showing a contactless thermostatic temperature controlling means physically embodied in the circuit of Figure 1 and illustrating the use of a modulating fuel controlling valve for providing modulation of the fuel flow under the control of said thermostatic temperature controlling means, and

Figure 4 is a fragmentary diagram illustrating another form of the invention utilizing temperature responsive means including a bimetallic temperature responsive element for controlling the flow of current through an electron tube, and hence through a circuit controlled by the current flow through said tube;

Referring first to the embodiment of the invention shown in Figure 1, the burner indicated at 1 is a main burner. It maybe the main burner for a room or'space heater, or it may be the burner for a water heater or for a floor furnace, an'oven burner, one or more top burners for a gas range, or any other burner, and of any suitable construction.

A fuel supply pipe or conduit 2 leads to the burner 1 for the delivery of gaseous or other fuel thereto, for example, through a mixing chamber 3 to which air is admitted through adjustable air inlets (not shown), as well understood in the art. The flow of gas through the conduit 2 is controlled by a valve '4 of any suitable electroresponsive or electrically operated type. For purposes of illustration,the valve 4' may be a solenoid valve operated to open position by the flow of electric current through its coil and to closed position by a spring.

interposed in the fuel supply pipe 2 anterior ofvalve 4- is a thermoelectric safety shut-off valve 8; The thermoelectric safety shut-ofi valve 8 may be, in general, of the type shown. and described in Clarence Wantz Patent No. 2,307,870, issued January 12, 1943. Suflice it to state that it comprises a valve body having an inlet at 9 for receiving gaseous or other fuel and an outlet 10 through which this fuel is delivered to the valve 4 and thencepwhen valve 4 is open, to the main burner. Con tiguous sections of pipe 2 are connected to inlet 9 and outlet 10. A passage, shown more or less diagrammati cally in dotted lines at 11, provides for communication between inlet 9 and outlet 10, and a safety shut-off valve, shown in dotted lines at 12, is adapted to seat, for. example, upon a valve seat at one end of passage 11 to shut off the flow of fuel for the main burner; also, the flow of fuel for a pilot burner 13 located in juxtaposition or in lighting proximity to the burner 1.

An el'ectromagnet having a magnet frame 14 and a coil 15 is adapted for holding armature 16 connected to valve 12 in attracted position and thereby the valve 12 open when the thermoelectric generatorsuch as athermocouple or thermopileis heated by the pilot flame. When the pilot 13 is extinguished, the electromagnet which is connectedin circuit with the thermoelectric generator, ceases to hold the valve 12 open, and this valve moves to closed position, for example, under the action of a spring 18 to shut d the supply of fuel to burner 1; also the supply of fuel to the pilot burner.

A reset button 19 is adapted to be pressed inwardly to move valve 12 to open position with accompanying movement of armature 16 to attracted position and to hold the armature in attracted position until the pilot is ignited and heats the thermoelectric generator 17 sufficiently to retain armature 16 attracted and valve 12 open. A flow interrupter valve may be provided for shutting off the flow of fuel to the main burner during the resetting operation and until the valve 12 is held in open position by the electromagnet and button 19 is released and returned to its outwardly projected position with accompanying operation of the flow interrupter valve 20 to open position. The details and mode of operation of a suitable reset and flow interrupter valve arrangement are shown and described in the above identified patent of Clarence Wantz; also in Charles V. Hildebracht Patent No. 2,114,446, issued April 19, 1938.

The pipe 21 for supplying fuel to the pilot burner 13 may be connected, for example, as shown more or less diagrammatically at 22 to the interior of the body of the thermoelectric safety shut-off valve 8 between the above-mentioned valve seats at opposite ends of the passage 11. In this way the valve 12, when in closed position, shuts off the supply of fuel to both the main burner 1 and the pilot burner 13, and fuel flows to the pilot burner 13 Whenever the valve 12 is open and regardless of whether the flow interrupter valve 21) is or is not open.

Electric power for operating the valve 4 is supplied from a suitable source, for example, from the line wires 23 of a suitable source of alternating current, such as a household current supply line of the type which averages about 115 volts. Power for the operation of the valve 4 may be supplied, for example, by the secondary winding 24 of a transformer having a primary winding 25 connected to the line Wires 23.

One side of a secondary 31 is connected by a conductor 32 to a rectifier 33 at 34. The other side of secondary 31 is connected by a conductor 35 to rectifier 33 at 36. One side of rectifier 33 is connected by a conductor 37 to the anode 38 of an electron tube 39 shown in the form of a diode. The cathode 40 of tube 39 is connected to the grid 62 of a thyratron 61 which in turn is connected to the grid 63 of a thyratron 60 by a conductor 64. The conductor 65, which is connected to the side of rectifier 33 opposite the side to which conductor 37 is connected, is connected to the negative side of a bias cell or battery 65. The positive side of cell 65 is connected through conductors 66, 67, and 68 and resistors 69 and 70 to anode 71 in parallel with cathode 75 and anode 76 in parallel with cathode 72.

One side of secondary 24 is connected by a conductor 73 to one side of the coil of the solenoid valve 4. The other side of secondary 24 is connected by a conductor 74 to cathode 75 of thyratron 61. The anode 76 of thyratron 61 is connected by a conductor 77 to the other side of the coil of the solenoid valve 4. The system also has a resistor shown at 78 connected between the conductors 41 and 65. The tubes 60 and 61 are shown as provided with heaters indicated diagrammatically at 80.

It is to be understood that while the control system disclosed herein and under the control of the current flow through the electron tube 39 utilizes thyratrons, other equivalent control systems, not utilizing thyratrons, may be substituted therefor. Examples of other forms of control systems which may, within the concept of the invention, be utilized in place of the herein disclosed control system, are forms having a relay or a saturable core reactor embodied therein.

The rectifier 33 energizes a coil 43 disposed in proximity to the tube 39 and connected at one end by a conductor 44 to conductor 37 between the rectifier and tube. The other end of coil 43 is connected to a conductor through an adjustable control 46. A third secondary 47 has its opposite sides connected by conductors 48 and 49 to a heater 50 for the cathode 40. When coil 43 is energized without energization of a coil 51 to be presently described, the coil 43 directs magnetic flux lines across the electron travel from cathode 40 to anode 38 to prevent the tube 39 from conducting.

A coil 51 is located physically on the outside of the vacuum tube 39 and connected electrically so that the magnetic field produced by flow of current through it opposes that produced by coil 43. The coil 51 is connected in series circuit relation with the elements of a thermoelectric generator 52 (thermocouple or thermopile) which is disposed so that the hot junction 53 thereof (or hot junctions) is subject to the heat of the pilot flame as shown.

In operation, cocking of the thermoelectric safety shut off valve 3 by depressing the buttons 19 opens the valve 12, thus admitting gas to the pilot burner 13 which, when ignited, heats the hot junctions of the thermoelectric generators 17 and 52. In cocking or resetting the thermoelectric safety shut-off valve, the flow interrupter valve member 20 moves to closed position to shut off the flow of fuel to the main burner 1 during the resetting operation and until the pilot 13 is ignited and heats the thermoelectric generator 17 sufficiently to hold valve 12 open. Then upon release of button 19, valve member 2i) moves to open position, admitting gas to solenoid valve 4.

When the pilot burner 13 is ignited, the flame thereof also heats the hot junction of the thermoelectric generator 52, thus setting up a flow of thermoelectric current through the coil 51. Line voltage applied to the primary 25 induces a voltage in secondaries 31 and 47 and applies voltage to the elements of diode 39. Coil 51 is, as previously stated, physically located on the outside of diode 39 and is connected electrically so that the magnetic field produced by the flow of current set up through it by the heat of the pilot burner on the hot junction of the thermoelectric generator 52 opposes that produced by the coil 43. The rectifier 33 also supplies plate voltage to the diode 39, and when coil 51 is energized, the diode conducts and causes current to flow through the conductor 41.

The output of diode 39 is fed through the resistor 78, and the flow of current through resistor 78 produces a positive bias on the grids of thyratrons 6t and 61 which are connected up back-to-back. The application of the positive bias causes the thyratrons to conduct, each one conducting when the plate is positive with respect to the cathode. This means that each tube conducts only dur ing one-half of the cycle, causing the fiow of alternating current through solenoid valve 4 which, being an AC. valve, is opened and held open for the passage of fuel to the main burner 1.

The purpose of the bias cell 65 in the grid circuit is to maintain a negative potential on the grids which prevents the thyratrons from firing when no current is flowing through the resistor 78, this being the condition that exists when the thermoelectric generator 52 is cold and the coil 51 is deenergized.

Figure 2 shows the magnetic diode 39 surrounded by its control winding 43, and a magnetic bimetal strip 92 is arranged so that a piece of soft iron 93 is moved into and out of the vicinity of the end of the magnetically controlled diode 39, thereby controlling the fiow of current through the diode. The movement of the bimetal strip 92 which is caused by ambient temperatures surrounding it, is used in place of a room thermostat. Changes in room temperature will cause a movement of the bimetal 92 in such a direction as to operate the electroresponsive valve 4, thereby causing the heat supplied by the main burner 1 to compensate for changes in room temperature surrounding the bimetal 92.

Thermostatic control of the flow of current through the tube 39 and hence control of the electroresponsive valve is accomplished as follows: Energization of the coil 43 directs magnetic flux lines across the electron travel from the cathode 40 to anode 38, which flux lines tend to prevent the tube 39 from conducting. Energization of the coil 51 by current from the thermocouple 32 produces a magnetic field which opposes the magnetic field produced by energization of the coil 43. The structures of the flux-directing coils 43 and 51 are such that the field produced by energization of the coil 43 has a greater number of ampere-turns than does the field produced by energization of the coil 51. A resultant field is produced which is equal in ampere-turns to the difference between the ampere-turns of the coils 43 and 51. This resultant field tends to prevent the tube from conducting, but it is completely effective in this respect only when the soft iron 93 is disposed by the bimetal 92 close to the diode 39 to thereby provide a minimum air gap between the soft iron 93 and the base plate 94. The iron 93, bimetal 92 and the base plate 94 provide a low reluctance flux path for the resultant field, and when the air gap between the plate 94 and iron 93 is at a minimum, the maximum flux can flow through said gap and hence through the tube 39. This maximum flux flow is effective to prevent the tube from conducting. However, when the bimetal 92 moves the soft iron away from the diode 39, for example in response to a predetermined drop in temperature, a relatively large air gap results, which gap increases the reluctance of'the flux path provided by the soft iron 93, bimetal 92 and base plate 94'. This renders the aforementioned resultant field ineffective to prevent the tube from conducting, and the tube therefore conducts, permitting current fio-w to the control circuit, causing energization and opening of the electroresponsive valve 4 to permit fuel flow to the burner 1.

Subsequent movement of the soft iron 93 toward the tube 39, for example in the response of the predetermined rise in temperature, decreases the air gap, and the resultant field again prevents the tube 39 from conducting. The electroresponsive valve 4 is thereby deenergized and closes to shut off the flow of fuel to the burner 1.

Extinguishment of the pilot burner 13 removes the heat from the hot junction of the thermoelectric generator 52 which cools, thereby decreasing the flow of current to the coil 51. This decreases the magnetic field of the coil 51, and the magnetic field of the coil 43 takes precedence and cuts off the flow of current through the tube 39, regardless of the position of the soft iron 93, thereby deenergizing and closing the electroresponsive valve 4 to shut off the flow of fuel to the main burner 1.

Extinguishment of pilot burner 13 also removes the heat from the hot junction of the thermoelectric generator 17 which cools and thereby discontinues or decreases the flow of current to the coil 15 of the thermoelectric safety shut-off valve 8. Valve 8 thereby operates to closed position, shutting off the flow of fuel to the pilot burner 13, and also providing a safety shut-off for shutting ofi the supply of fuel to the electroresponsive valve 4.

The system shown in Figure 3 illustrates the use of the system of Figures 1 and 2 for the control of the modulating type valve 4' to provide modulation of the fuel flow to the burner 1' in accordance with changes in ambient temperature sensed by the bimetal 92.. With the exception of the valve 4', the parts shown in Figure 3 are the same as the parts shown in Figures 1 and 2, and are designated by primed reference characters corresponding to those shown in Figures 1 and 2. In Figure 3 both of the coils 43 and 51 are wound about the diode 39. The electroresponsive valve 4' may be of any suitable type which is provided with an operator 6' which holds said valve in closed position when said operator is deenergized, and opens said valve to varying degrees in accordance with the intensity of the current with which said operator is energized. Thus, when the maximum amount of current which the tube 39' is capable of conducting flows through said tube, the operator 6' holds the valve 4' in wide open position. For intermediate values of energizing current the operator 6 holds the valve 4' in corresponding intermediate open positions.

The movement of the soft iron 93 toward and away from the tube 39' in response to changes in ambient temperature sensed by the bimetal 92' varies the amount of current flowing through the tube 39', the amount of current flowing through said tube varying directly with the air gap between the soft iron 93 and the base plate 94. Assuming that the soft iron 93 is positioned close to the tube 39 to thereby prevent said diode from conducting, the operator 6' of the valve 4 is deenergized, and said valve is thereby closed. If desired, the valve 4' may be provided with a bypass permitting limited fuel flow to the burner 1 when said valve is closed, as is well known in the modulating valve art. As the temperature of the room drops, the bimetal 92 gradually moves the soft iron 93' away from the diode 39', decreasing the effect of the resultant field upon the flow of electrons from the cathode as to the anode 38', and permitting a small amount of current to flow from said cathode to said anode. This limited current flow opens the valve 4' a correspondingly small amount to permit fuel flow to the main burner 1 If the room temperature continues to drop, the bimetal 92' gradually moves the soft iron 93 farther away from the tube, and said tube thereupon conducts an increased amount of current to thereby cause the valve 4' to open a greater amount. This, of course, produces a greater amount of fuel flow to the burner 1', and the heat output of the burner 1 eventually matches to the heat loss from the room in which the bimetal 92' is located. If the heat output of the burner 1' exceeds the heat loss of the room, the temperature of said room rises, and the bimetal thereupon moves the soft iron 93 toward the tube 39' to reduce the current flow to the tube and correspondingly reduce the opening of the valve 4'. This, of course, reduces the amount of fuel flow to the burner 1 and hence the heat output of said burner.

It is apparent, therefore, that the system shown in Figures 1 and 2 is well adapted for use in controlling the operation of a modulating type valve, for example the valve 4' shown in Figure 3, to provide modulation of fuel flow to a burner in accordance with changes. in the ambient temperature.

Figure 4 illustrates another thelmostatic means for electromagnetically controlling the flow of current through an electron tube, said means being adapted for use in either of the systems shown in Figures 1 and 3. The parts indicated in Figure 4 having reference characters with the suflix a correspond to similar parts in Figures 1 to 3 indicated by the same reference characters primed or unprimed. In Figure 4, the soft iron 93 and 93 of Figures 2 and 3 is replaced by a permanent magnet 93a, and the coil 43 and 43 of Figures 1 and 3 is omitted. it is understood, of course, that the disclosure of Figure 4 is only fragmentary, and in order to: provide a complete system, the conductors having broken ends should be connected into a suitable control system, for example such as those shown in Figures 1 and 3. In making the connection, the conductors of Figure 4 having broken ends are connected to the conductors of Figures 1 or 3 bearing the same reference characters.

The operation of the form of the invention shown in Figure 4 is as follows. The permanent magnet 93a clirects magnetic flux lines across the electron travel from cathode 49a to anode 38a and tends to prevent the tube 39a from conducting. The magnetic field produced by the flow of thermoelectric current through the coil 51a opposes that produced by the permanent magnet 93a, but has an ampere-turn value less than that of the field of the permanent magnet. The resultant field tends to prevent a diode 39 from conducting but is only effective in this respect when the air gap between the permanent magnet 93a and the base plate 94a is at a minimum. As the permanent magnet 93a is moved away from the tube 7 39a by the bimetal 92a, the eifect of the resultant field in preventing conduction of current by the tube 39a is decreased, so that current can flow from the cathode 40a to the anode 38a. The amount of current which the tube can conduct varies directly with the air gap between the magnet 93a and the base plate 94a, said conduction being maximum when the air gap is maximum. In the latter instance the effect of the resultant field becomes zero as the opposing fields of the permanent magnet 93a and coil 51a balance each other.

In the event of pilot burner outage, the thermocouple 52a cools, and the coil 51a becomes deenergized, whereupon the field of the magnet 93a takes precedence and cuts off the current flow to the tube 39a regardless of the air gap between the permanent magnet and the base plate 94a.

The form of the invention shown in Figure 4 can be utilized to control an on-oif type electroresponsive valve or a modulating type electroresponsive valve, depending upon whether the systems of Figuers l or 3 are utilized therewith.

The embodiments of the invention shown in the drawings are for illustrative purposes only and it is to be expressly understood that said drawings and the accompanying specification are not to be construed as the definition of the limits or scope of the invention, reference being had to the appended claims for this purpose.

What is claimed as the invention is:

1. Control apparatus comprising, an electron tube having an anode and a cathode; means for energizing said tnbe to produce a flow of electrons from said cathode to said anode; a control circuit connected to said anode and cathode and under the control of the current flow from said cathode to said anode; first magnetic flux directing means for directing across the electron travel from said cathode to said anode magnetic flux lines tend ing to prevent the tube from conducting electric current; second magnetic flux directing means for directing across the electron travel from said cathode to said anode magnetic flux lines in opposition to the magnetic flux lines directed by said first magnetic flux directing means, thereby producing a resultant flux tending to prevent the tube from conducting; electric energy source means for energizing said second flux directing means and including a portion subject to the heat of burning fuel, said source means being operative to energize said second flux directing means when said portion is heated, and said flux directing means being deenergized on outage of said burning fuel, condition responsive means affording a low reluctance path for said resultant flux, and having therein a high reluctance gap variab e in accordance with the condition to thereby vary the amount of resultant flux directed across the electron travel between said cathode and said anode and hence the amount of current conducted by said tube: and a control device in said control circuit having a flow-controlling member movable between flow-permitting and flow-preventing positions in accordance with the amount of current conducted by said tube and hence in accordance with the condition, said first flux directing means pre venting said tube from conducting and thereby efiecting disposition of said flow controlling member in flow-preventing position on deenergization of said second flux directing means resulting from outage of said burning fuel, thereby affording safety shut-off of thc apparatus.

2. in a control a paratus: An electron tube having an anode and a cathode; means for en gizing said tube to produce a flow of electrons from said cathode to said anode; a control circuit connected to said anode and cathode and under the control of the current fiow from said cathode to said anode; first magnetic flux directing means for directing across the electron travel from said cathode to said anode magnetic flux lines tending to prevent the tube from conducting electric cur rent; second magnetic flux directing means for directing across the electron travel from said cathode to said anode magnetic flux lines in opposition to the magnetic flux lines directed by said first magnetic flux directing means, thereby producing a resultant flux tending to prevent the tube from conducting; electric energy source means for energizing said second flux directing means and including a portion subject to the heat of burning fuel, said source means being operative to energize said second flux directing means when said portion is heated, and said iluz; directing means being deeuergized on outage of said burning fuel, temperature responsive means affording a low reluctance path for said resultant flux, and having therein a high reluctance gap variable in accordance with the temperature to thereby vary the amount of resultant flux directed across the electron travel between said cathode and said anode and hence the amount of current conducted by said tube; and a fuel modulating control device in said control circuit having a flow-controlling member modulated between flow-permitting and flow-preventing positions in accordance with the amount of current conducted by said tube and hence in accordance with the temperature, said first flux directing means preventing said tube from conducting and thereby effecting disposition of said fiow controlling member in flow-preventing position on deenergization of said flux directing means resulting from outage of said burning fuel, thereby affording safety shut-off of the apparatus.

3. Control apparatus comprising, a control device having a flow-controlling member movable between flowpermitting and flow-preventing positions in response to energization and deenergization of said device, respectively, an electrical circuit controlling the operation of said device and including an electron tube, said circuit affording energization of said device when said tube is conducting and deenergization thereof when said tube is not conducting, first means for producing a magnetic field adjacent said tube tending to prevent the latter from conducting, second means for producing when energized a second magnetic field adjacent said tube in opposition to the field produced by said first means, thereby producing a resultant flux tending to prevent said tube from conducting, flame sensitive energy source means for energizing said second means when a flame is sensed thereby and deenergizing said second means on flame outage, condition responsive means operatively associated with said tube for varying the effectiveness of said resultant flux and hence the amount of current conducted by said tube and the corresponding position of said flow-controlling member in accordance with changes in the condition, said first means preventing said tube from conducting and thereby effecting disposition of said flow-controlling member in flow-preventing position on deenergization of said second means resulting from flame outage, thereby aflording safety shut ofi of the apparatus.

4. Control apparatus comprising, a modulating control device having a flow-controlling member modulated between flow-permitting and flow-preventing positions in accordance with the degree of energization and deenergization of said device, respectively, an electrical circuit controlling the operation of said device and including an electron tube, said circuit aifording energization of said device in accordance with the amount of current conducted by said tube when said tube is conducting and deenergization thereof when said tube is not conducting, first means for producing a magnetic field adjacent said tube tending to prevent the latter from conducting, second means for producing when energized a second magnetic field adjacent said tube in opposition to the field produced by said first means, thereby producing a resultant flux tending to prevent said tube from conducting, fiame sensitive energy source means for energizing said second means when a flame is sensed thereby and deenergizing said sec- References Cited in the file of this patent UNITED STATES PATENTS Thomson Sept. 11, 1888 Hewitt May 30, 1922 MacGahan May 3, 1927 Machlet Oct. 6, 1936 Ballantine July 4, 1939 Linder Oct. 29, 1940 Ziebolz Oct. 7, 1947 Glass Dec. 16, 1947 Hunter a Apr. 20, 1948 

